1. Field of the Invention
The present invention relates to a coding apparatus and a decoding apparatus for executing orthogonal transform (DCT (Discrete Cosine Transform), in particular), which can code signals and decode the coded signals by a high efficient coding and decoding method on the basis of a small coding quantity, in a digital signal processing (e.g., recording, transmitting, displaying, etc.) system.
2. Description of the Prior Art
In the high efficient coding technique using the orthogonal transform such as DCT, the quantity of data can be reduced on the basis of effective utilization of interframe correlation. Therefore, this coding technique is widely adopted as the standard technique. In this coding technique, the orthogonal transform is executed, in general, in two dimensions of vertical and horizontal directions of a block of image signals, for instance. This is because in the case of the one dimensional orthogonal transform, only one of the vertical and horizontal interframe correlations can be used; on the other hand, in the case of the two dimensional orthogonal transform, both the vertical and horizontal interframe correlations can be used.
Further, it is effective to increase the size of transform block from the correlation standpoint, because the interframe correlation can be utilized more efficiently with increasing size of the transform block. In practice, however, there exists no substantial difference when the number of pixels is 8.times.8 or more. On the other hand, since the quantization error extends over the block, it is preferable to decrease the size of the block from the visual standpoint. In addition, the processing quantity decreases with decreasing size of the block.
In summary, it is most general to transform image signals of the block of 8.times.8 pixels (8 degrees in both vertical and horizontal directions). In addition, in the case of the moving image coding, it is general to execute interframe predictive coding in the time direction and to adopt the spatial orthogonal transform for the prediction error.
FIG. 1 shows a prior art coding apparatus. In the drawing, the apparatus comprises an image input 1, a horizontal DCT (discrete cosine transformer) 2, a vertical DCT 3, a quantizer 5, a variable length coder 6, and a coded signal output 7. The horizontal DCT 2 transforms inputted image signals in accordance with 8-degree discrete cosine transform in the horizontal direction for each two-dimensional block of 8.times.8 pixels. Further, the vertical DCT 3 further transforms the horizontally DCTed (discrete cosine transformed) signals in accordance with 8-degree discrete cosine transform in the vertical direction. The quantizer 5 quantizes the DCTed signals to such a quantization step width that the quantization error is not distinguishable visually. Here, the coefficients of almost all quantized signals are zero. The variable length coder 6 transforms the sequence of signals arranged in the two-dimensional block status into the sequence of signals arranged in the one-dimensional status (referred to as "zigzag scanning sequence") as listed in FIG. 2A. The run lengths of the zero coefficients and the values of the non-zero coefficients are coded in accordance with a VLC (variable length coding) such as Huffuman code. The output of the variable length coder 6 is outputted as compressed data to a decoding apparatus (described below) through the code output 7.
FIG. 3 shows a prior art decoding apparatus corresponding to the coding apparatus shown in FIG. 1. The decoding apparatus comprises a coded signal input 21, a variable length decoder 22, an inverse quantizer 23, a vertical inverse DCT 24, a horizontal inverse DCT 25, and an image output 26. The variable length decoder 22 decodes the compressed data of the variable length code to the fixed length codes. The inverse quantizer 23 inversely quantizes the fixed length codes to values representative of the quantization of the coded signals. The vertical inverse DCT 24 inversely transforms the inputted representative values in the vertical direction. The horizontal inverse DCT 25 further inversely transforms the inputted representative values in the horizontal direction. The obtained reproduced image signals are outputted through an image signal output 26.
In the above-mentioned two-dimensional orthogonal transform coding apparatus, the transform efficiency is relatively high in the image portion in which the interframe correlation is relatively high. However, the prior art apparatus is not necessarily suitable for the image portion in which the interframe correlation is relatively low, for instance at the image edge portions. The above-mentioned tendency is prominent in the interframe prediction error signals, in particular. In other words, since the interframe correlation of the error signals is low, the two-dimensional orthogonal transform is not an appropriate coding method. Therefore, there exist some portions at which the one-dimensional transform such as only the horizontal or vertical transform is higher in efficiency than the two-dimensional transform.
Further, in the two-dimensional transform, the quantization error extends over the two-dimensional block, so that the noise components are prominent in the vicinity of the image edges. In other words, the one-dimensional transform using a small transform block is preferable from the visual standpoint, as far as the equivalent coding error is allowed. Further, although the other coding technique such as DPCM (Differential Pulse Code Modulation) is preferable because the noise components are not prominent from the visual standpoint, the interframe correlation cannot be utilized effectively in the other coding technique, so that the basic efficiency is insufficient.